The present invention relates generally to position detection apparatus and particularly to apparatus for determining the position of a member that is movable along a defined path of finite length.
It is a common problem to want to know the position of a device whose position is being controlled by an actuator or some other means. For example, in the controls industry, devices such as valves having a valve stem or valve shaft which is movable by an actuator are used to control the flow of liquids and gasses associated with industrial processes of various types. In these applications it is common to want to know, at any given time, the precise position of the movable valve stem or valve shaft. This information allows improved understanding of the process and the control of the process.
A number of prior solutions have been proposed. Optical coding schemes make use of a coded element with opaque and transparent sections to provide digital data inputs to an array of sensors positioned to measure the light passing through the sections. While optical coding devices do not require a mechanical linkage, the optical approach only works well in very clean environments and is therefore not applied in many industrial environments.
Linear variable differential transformers (lvdt) can provide very accurate position information. However they require a mechanical linkage and also generally use relatively high power.
Potentiometers or other rotary transducers require a mechanical linkage and also have the disadvantage of a sliding electrical contact which can cause long term reliability issues.
Hall effect transducers, as they are currently used, generally require a mechanical linkage.
Thus a need exists for a reliable position determining apparatus that does not require a mechanical linkage between the apparatus and the movable member, and is relatively insensitive to environmental factors.